Radially expandable polymer prosthesis and method of making same

ABSTRACT

Polymeric stents having fracture toughness and resistance to recoil after deployment are disclosed along with methods of manufacturing such stents. Improvements to mechanical characteristics and other improvements may be achieved by having polymer chains within individual stent struts oriented in a direction that is closer to or in line with the axis of the individual stent struts. The struts are connected to each other by hinge elements that are configured to bend during crimping and deployment of the stent. Ring struts form ring structures. A ring structure can have an overall curvilinear length from about 12 mm to about 15 mm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/831,440,filed Jul. 10, 2010, which claims the benefit of Provisional ApplicationNo. 61/323,789, filed Apr. 13, 2010 and is a continuation-in-part ofapplication Ser. No. 12/114,608, filed May 2, 2008, now U.S. Pat. No.8,002,817, which claims the benefit of Provisional Application No.60/927,785, filed May 4, 2007, all of which applications areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to expandable endoprostheses, and moreparticularly to polymeric stents and methods of manufacturing polymericstents.

BACKGROUND OF THE INVENTION

An “endoprosthesis” corresponds to an artificial device that is placedinside the body, more particularly, within an anatomical lumen. A“lumen” refers to a cavity of a tubular organ such as a blood vessel.

A stent is an example of such an endoprosthesis. Stents are generallycylindrically shaped devices, which function to hold open and sometimesexpand a segment of a blood vessel or other anatomical lumen such asurinary tracts and bile ducts. Stents are often used in the treatment ofatherosclerotic stenosis in blood vessels. “Stenosis” refers to anarrowing or constriction of the diameter of a bodily passage ororifice. In such treatments, stents reinforce body vessels and preventrestenosis following angioplasty in the vascular system. “Restenosis”refers to the reoccurrence of stenosis in a blood vessel or heart valveafter it has been treated (as by balloon angioplasty, stenting, orvalvuloplasty) with apparent success.

The treatment of a diseased site or lesion with a stent involves bothdelivery and deployment of the stent. “Delivery” refers to introducingand transporting the stent through a bodily lumen to a region, such as alesion, in a vessel that requires treatment. “Deployment” corresponds tothe expanding of the stent within the lumen at the treatment region.Delivery and deployment of a stent are accomplished by positioning thestent about one end of a catheter, inserting the end of the catheterthrough the skin into a bodily lumen, advancing the catheter in thebodily lumen to a desired treatment location, expanding the stent at thetreatment location, and removing the catheter from the lumen.

In the case of a balloon expandable stent, the stent is mounted about aballoon disposed on the catheter. Mounting the stent typically involvescompressing or crimping the stent onto the balloon. The stent is thenexpanded by inflating the balloon. The balloon may then be deflated andthe catheter withdrawn. In the case of a self-expanding stent, the stentmay be secured to the catheter via a retractable sheath or a sock. Whenthe stent is in a desired bodily location, the sheath may be withdrawnwhich allows the stent to self-expand.

The stent must be able to satisfy a number of mechanical requirements.First, the stent must be capable of withstanding the structural loads,namely radial compressive forces, imposed on the stent as it supportsthe walls of a vessel. Therefore, a stent must possess adequate radialstrength. Radial strength, which is the ability of a stent to resistradial compressive forces, is due to strength and rigidity around acircumferential direction of the stent. Radial strength and rigidity,therefore, may also be described as, hoop or circumferential strengthand rigidity.

Once expanded, the stent must adequately maintain its size and shapethroughout its service life despite the various forces that may come tobear on it, including the cyclic loading induced by the beating heart.For example, a radially directed force may tend to cause a stent torecoil inward. Generally, it is desirable to minimize recoil.

In addition, the stent must possess sufficient flexibility to allow forcrimping, expansion, and cyclic loading. Longitudinal flexibility isimportant to allow the stent to be maneuvered through a tortuousvascular path and to enable it to conform to a deployment site that maynot be linear or may be subject to flexure. Finally, the stent must bebiocompatible so as not to trigger any adverse vascular responses.

The structure of a stent is typically composed of scaffolding thatincludes a pattern or network of interconnecting structural elementsoften referred to in the art as struts or bar arms. The scaffolding canbe formed from wires, tubes, or sheets of material rolled into acylindrical shape. The scaffolding is designed so that the stent can beradially compressed (to allow crimping) and radially expanded (to allowdeployment). A conventional stent is allowed to expand and contractthrough movement of individual structural elements with respect to eachother.

Additionally, a medicated stent may be fabricated by coating the surfaceof either a metallic or polymeric scaffolding with a polymeric carrierthat includes an active or bioactive agent or drug. Polymericscaffolding may also serve as a carrier of an active agent or drug.

Furthermore, it may be desirable for a stent to be biodegradable. Inmany treatment applications, the presence of a stent in a body may benecessary for a limited period of time until its intended function of,for example, maintaining vascular patency and/or drug delivery isaccomplished. Therefore, stents fabricated from biodegradable,bioabsorbable, and/or bioerodable materials such as bioabsorbablepolymers should be configured to completely erode only after theclinical need for them has ended.

However, there are potential shortcomings in the use of polymers as amaterial for implantable medical devices, such as stents. There is aneed for a manufacturing process for a stent that addresses suchshortcomings so that a polymeric stent can meet the clinical andmechanical requirements of a stent.

Polymers have been used to make stent scaffolding. The art recognizes avariety of factors that affect a polymeric stent's ability to retain itsstructural integrity when subjected to external loadings, such ascrimping and balloon expansion forces. These interactions are complexand the mechanisms of action not fully understood. According to the art,characteristics differentiating a polymeric, bio-absorbable stentscaffolding of the type expanded to a deployed state by plasticdeformation from a similarly functioning metal stent are many andsignificant. Indeed, several of the accepted analytic or empiricalmethods/models used to predict the behavior of metallic stents tend tobe unreliable, if not inappropriate, as methods/models for reliably andconsistently predicting the highly non-linear behavior of a polymericload-bearing, or scaffolding portion of a balloon-expandable stent. Themodels are not generally capable of providing an acceptable degree ofcertainty required for purposes of implanting the stent within a body,or predicting/anticipating the empirical data.

Polymer material considered for use as a polymeric stent scaffolding,such as PLLA and PLGA, may be described through comparison with ametallic material conventionally used to form stent scaffolding. Incomparison to metals, a suitable polymer has a low strength to weightratio, which means more material is needed to provide an equivalentmechanical property to that of a metal. Therefore, struts in polymericscaffolding must be made thicker and wider to have the strength needed.Polymeric scaffolding also tends to be brittle or have limited fracturetoughness. The anisotropic and rate-dependant inelastic properties(i.e., strength/stiffness of the material varies depending upon the rateat which the material is deformed) that are inherent in the materialonly compound this complexity in working with a polymer, particularly, abio-absorbable polymer such as PLLA and PLGA.

Therefore, processing steps performed on and design changes made to ametal stent that have not typically raised concerns for unanticipatedchanges in the average mechanical properties, may not also apply to apolymer stent due to the non-linear and sometimes unpredictable natureof the mechanical properties of the polymer under a similar loadingcondition. It is sometimes the case that one needs to undertakeextensive validation before it even becomes possible to predict moregenerally whether a particular condition is due to one factor oranother—e.g., was a defect the result of one or more steps of afabrication process, or one or more steps in a process that takes placeafter stent fabrication, e.g., crimping. As a consequence, a change to afabrication process, post-fabrication process or even relatively minorchanges to a stent pattern design must, generally speaking, beinvestigated more thoroughly than if a metallic material were usedinstead of the polymer. It follows, therefore, that when choosing amongdifferent polymeric stent designs for improvement thereof, there are farless inferences, theories, or systematic methods of discovery available,as a tool for steering one clear of unproductive paths, and towards moreproductive paths for improvement, than when making design changes in ametal stent.

It is recognized, therefore, that, whereas inferences previouslyaccepted in the art for stent validation or feasibility when anisotropic and ductile metallic material was used, such inferences wouldbe inappropriate for a polymeric stent. A change in a polymeric stentpattern may affect, not only the stiffness or lumen coverage of thestent in its deployed state, but also the propensity for fractures todevelop when the stent is crimped or being deployed. This means that, incomparison to a metallic stent, there is generally no assumption thatcan be made as to whether a changed stent pattern may not produce anadverse outcome, or require a significant change in a processing step(e.g., tube forming, laser cutting, crimping, etc.). Simply put, thehighly favorable, inherent properties of a metal (generally invariantstress/strain properties with respect to the rate of deformation or thedirection of loading, and the material's ductile nature), which simplifythe stent fabrication process, allow for inferences to be more easilydrawn between a changed stent pattern and/or a processing step and theability for the metallic stent to be reliably manufactured with the newpattern and without defects when implanted within a living being.

A change in the pattern of the struts and rings of a polymeric stentscaffolding that is plastically deformed, both when crimped to, and whenlater deployed by a balloon, unfortunately, is not as easy to predict asa metal stent. Indeed, it is recognized that unexpected problems mayarise in polymer stent fabrication steps as a result of a changedpattern that would not have necessitated any changes if the pattern wasinstead formed from a metal tube. In contrast to changes in a metallicstent pattern, a change in polymer stent pattern may necessitate othermodifications in fabrication steps or post-fabrication processing, suchas crimping and sterilization.

A problem encountered with polymeric stents after they are crimped ontoa balloon is the development of fractures and other defects that requirethe stent to be rejected and scrapped. Cracks and other defects canrender the stent incapable of functioning properly when fully deployedby the balloon. Another problem is that deployment of polymeric stentsfrom the crimped state to a deployed state in a patient can producestrain that adversely affects the ability of the stent to stay in thedeployed state and remain at the implantation site, especially undercyclic loading conditions inherent in a patient's circulatory system.The strain induced during deployment can result in significant loss inradial strength.

In light of the foregoing, there is a need for a stent pattern andmanufacturing method that reduces the cracks and other defects due tocrimping and/or deployment. There is also a need for a stent pattern andmanufacturing method that results in less strain when a stent isdeployed for implantation.

SUMMARY OF THE INVENTION

Briefly and in general terms, the present invention is directed to anendoprosthesis and a method of making an endoprosthesis.

In aspects of the present invention, an endoprosthesis comprises atubular network of struts cut from a radially expanded PLLA polymertube. The tubular network comprises W-shaped closed cells, each W-shapedclosed cell abutting six other W-shaped closed cells, ach W-shapedclosed cell comprising ring struts, link struts, U-shaped hingeelements, and Y-shaped hinge elements. The ring struts form a pluralityof ring structures, each ring structure connected to another one of thering structures by at least one link strut. Each U-shaped hinge elementconnects exactly two ring struts to each other, the U-shaped hingeelement being tangent to the two ring struts. Each Y-shaped hingeelement connects a link strut to exactly two ring struts. The tubularnetwork has an outer diameter of about 3.5 mm and is configured to becrimped to an outer diameter of about 1.3 mm and expanded to an outerdiameter of about 3.8 mm without substantial damage to the U-shapedhinge elements.

In other aspect of the present invention, a method comprises cutting aradially expanded PLLA polymer tube to form a tubular network of struts.The tubular network comprises W-shaped closed cells. Each W-shapedclosed cell abuts six other W-shaped closed cells. Each W-shaped closedcell comprises ring struts, link struts, U-shaped hinge elements, andY-shaped hinge elements. The ring struts form a plurality of ringstructures. Each ring structure is connected to another one of the ringstructures by at least one link strut. Each U-shaped hinge elementconnects exactly two ring struts to each other, the U-shaped hingeelement being tangent to the two ring struts. Each Y-shaped hingeelement connects a link strut to exactly two ring struts. The tubularnetwork has an outer diameter of about 3.5 mm and is configured to becrimped to an outer diameter of about 1.3 mm and expanded to an outerdiameter of about 3.8 mm without substantial damage to the U-shapedhinge elements.

The features and advantages of the invention will be more readilyunderstood from the following detailed description which should be readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a stent.

FIG. 2 depicts a process flow diagram in accordance with an embodimentof the present invention.

FIG. 3 depicts a stent pattern viewed in a flat or planar state.

FIG. 4 depicts another stent pattern viewed in a flat or planar state.

FIG. 5 depicts a simplified view of the stent pattern of FIG. 4 in acylindrical state.

FIG. 6 depicts a detailed view of a portion of the stent pattern of FIG.4, showing rings in a non-deformed configuration, the rings having aninitial diameter.

FIG. 7 depicts a partial view of a ring from FIG. 4, showing the ring ina deformed configuration after being radially collapsed to a diameterless than the initial diameter.

FIG. 8 depicts a partial view of a ring from FIG. 4, showing the ring inanother deformed configuration after manufacturing, the ring having beendeployed at a diameter greater than the initial diameter.

FIG. 9 depicts a perspective view of a stent having the stent pattern ofFIG. 4.

FIG. 10 depicts a stent pattern viewed in a flat or planar state, thestent pattern having W-shaped cells having varying sizes in thecircumferential direction.

FIG. 11 depicts a stent pattern viewed in a flat or planar state, thestent pattern having W-shaped cells having varying sizes in the axialdirection.

FIG. 12 depicts a detailed view of a portion of the stent pattern ofFIG. 6, showing U-shaped hinge elements that are non-tangent to linearring struts.

FIG. 13 depicts a detailed view of a portion of a stent pattern, showingU-shaped hinge elements that are tangent to linear ring struts.

FIG. 14 depicts a photograph of a stent having U-shaped hinge elementsthat are non-tangent to linear ring struts, showing cracks and crazingat bend areas after crimping and deployment of the stent.

FIG. 15 depicts a photograph of a stent having U-shaped hinge elementsthat are tangent to linear ring struts, showing an absence of cracks andcrazing at bend areas after crimping and deployment of the stent.

FIG. 16 depicts another stent pattern viewed in a flat or planar state,the pattern having U-shaped hinge elements that are tangent to linearring struts.

FIG. 17 depicts a detail view of a rectangular region of the pattern ofFIG. 16, illustrating the curvilinear length of one W-shaped closed cellrepresentative of surrounding W-shaped closed cells.

FIG. 18 depicts a detail view of a rectangular region of the pattern ofFIG. 4, illustrating, for the purpose of comparison with FIG. 17, thecurvilinear length (CL) of one W-shaped closed cell representative ofsurrounding W-shaped closed cells.

FIGS. 19A and 19B are photographs of a stent in a crimped and expandedstate, the stent having a pattern of W-shaped closed cell with CL=4.0mm.

FIGS. 20A and 20B are photographs of a stent in a crimped and expandedstate, the stent having a pattern of W-shaped closed cell with CL=4.5mm.

FIGS. 21A and 21B are photographs of a stent in a crimped and expandedstate, the stent having a pattern of W-shaped closed cell with CL>4.5mm.

DETAILED DESCRIPTION OF THE INVENTION

The various embodiments of the present invention relate to polymericstents and methods of fabricating polymeric stents with favorablemechanical properties. The present invention can be applied to devicesincluding, but is not limited to, self-expandable stents,balloon-expandable stents, stent-grafts, and grafts (e.g., aorticgrafts).

FIG. 1 depicts a partial perspective view of an exemplary stent 100 thatincludes a pattern of a plurality of interconnecting structural elementsor struts. Stent 100 has a cylindrical shape with an axis 160 andincludes a pattern with a number of interconnecting structural elementsor struts 110. Axis 160 extends through the center of the cylindricalshape. In general, a stent pattern is designed so that the stent can beradially compressed to allow for percutaneous delivery through ananatomical lumen, then deployed for implantation at the desired segmentof the anatomical lumen. As used herein, deployment of the stent refersto radial expansion of the stent to implant the stent in the patient.The stresses involved during compression and deployment are generallydistributed throughout various structural elements of the stent pattern.

The underlying structure or substrate of stent 100 is typically theprimary source of the radial strength of the stent. The substrate can becompletely or at least in part made from a biodegradable polymer orcombination of biodegradable polymers, a biostable polymer orcombination of biostable polymers, or a combination of biodegradable andbiostable polymers. Additionally, a polymer-based coating applied overthe substrate can include a biodegradable polymer or combination ofbiodegradable polymers, a biostable polymer or combination of biostablepolymers, or a combination of biodegradable and biostable polymers.

Representative examples of polymers that may be used to fabricate orcoat an implantable medical device of the present invention include, butare not limited to, poly(N-acetylglucosamine) (Chitin), Chitosan,poly(hydroxyvalerate), poly(lactide-co-glycolide),poly(hydroxybutyrate), poly(hydroxybutyrate-co-valerate),polyorthoester, polyanhydride, poly(glycolic acid), poly(glycolide),poly(L-lactic acid), poly(L-lactide), poly(D,L-lactic acid),poly(D,L-lactide), poly(caprolactone), poly(trimethylene carbonate),polyester amide, poly(glycolic acid-co-trimethylene carbonate),co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes, biomolecules(such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronicacid), polyurethanes, silicones, polyesters, polyolefins,polyisobutylene and ethylene-alphaolefin copolymers, acrylic polymersand copolymers other than polyacrylates, vinyl halide polymers andcopolymers (such as polyvinyl chloride), polyvinyl ethers (such aspolyvinyl methyl ether), polyvinylidene halides (such as polyvinylidenechloride), polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics(such as polystyrene), polyvinyl esters (such as polyvinyl acetate),acrylonitrile-styrene copolymers, ABS resins, polyamides (such as Nylon66 and polycaprolactam), polycarbonates, polyoxymethylenes, polyimides,polyethers, polyurethanes, rayon, rayon-triacetate, cellulose, celluloseacetate, cellulose butyrate, cellulose acetate butyrate, cellophane,cellulose nitrate, cellulose propionate, cellulose ethers, andcarboxymethyl cellulose. Another type of polymer based on poly(lacticacid) that can be used includes graft copolymers, and block copolymers,such as AB block-copolymers (“diblock-copolymers”) or ABAblock-copolymers (“triblock-copolymers”), or mixtures thereof.

Additional representative examples of polymers that may be especiallywell suited for use in fabricating or coating an implantable medicaldevice include ethylene vinyl alcohol copolymer (commonly known by thegeneric name EVOH or by the trade name EVAL), poly(butyl methacrylate),poly(vinylidene fluoride-co-hexafluororpropene) (e.g., SOLEF 21508,available from Solvay Solexis PVDF, Thorofare, N.J.), polyvinylidenefluoride (otherwise known as KYNAR, available from ATOFINA Chemicals,Philadelphia, Pa.), ethylene-vinyl acetate copolymers, and polyethyleneglycol.

Stent 100 may be fabricated from a polymeric tube or a polymeric sheetthat has been rolled and bonded to form a tube. A stent pattern may beformed on the polymeric tube or sheet by laser cutting away portions ofthe tube or sheet, leaving only struts and other members that functionas scaffolding to support the walls of an anatomical lumen.Representative examples of lasers that may be used include, but are notlimited to, excimer, carbon dioxide, and YAG. In other embodiments,chemical etching may be used to form a pattern on a tube.

The pattern of stent 100 in FIG. 1 allows for radial expansion andcompression and longitudinal flexure. The pattern includes struts thatare straight or relatively straight, an example being a portion 120. Inaddition, patterns may include bending elements 130, 140, and 150.Bending elements bend inward when a stent is crimped to allow radialcompression of the stent in preparation for delivery through ananatomical lumen. Bending elements also bend outward when a stent isdeployed to allow for radial expansion of the stent within theanatomical lumen. After deployment, stent 100 is subjected to static andcyclic compressive loads from the vessel walls. Thus, bending elementsmay deform during use.

As indicated above, a stent has certain mechanical requirements. A stentmust have sufficient radial strength to withstand structural loads,namely radial compressive forces, imposed on the stent as it supportsthe walls of a vessel or other anatomical lumen. In addition, the stentmust possess sufficient flexibility to allow for crimping, deployment,and cyclic loading. Also, a sufficiently low profile, that includesdiameter and size of struts, is important. As the profile of a stentdecreases, the easier is its delivery, and the smaller the disruption ofblood flow.

Polymers tend to have a number of shortcomings for use as substratematerials for stents. Compared to metals, the strength to weight ratioof polymers is smaller than that of metals. A polymeric stent withinadequate radial strength can result in mechanical failure or recoilinward after implantation into a vessel. To compensate for therelatively low modulus of polymers as compared to metals, a polymericstent requires significantly thicker struts than a metallic stent, whichcan result in an undesirably large profile.

Another shortcoming of polymers is that many polymers, such asbiodegradable polymers, tend to be brittle under physiologicalconditions or conditions within a human body. Specifically, somebiodegradable polymers that have a glass transition temperature, Tg,above human body temperature of about 37° C. exhibit a brittle fracturemechanism in which there is little or no plastic deformation prior tofailure. As a result, a stent fabricated from such polymers can haveinsufficient toughness for the range of use of a stent. In particular,it is important for a stent to be resistant to fracture throughout therange of use of a stent, i.e., crimping, delivery, deployment, andduring a desired treatment period.

A potential problem with polymeric stents is mechanical creepdeformation. Creep refers to the gradual deformation of a structuresubjected to an applied load. It is believed that the delayed responseof polymer chains to stress causes creep by means of a phenomenon knownas reptation. Reptation occurs when un-branched polymer chains slip pastone another and away from the bulk entanglement in response to anapplied load. This behavior in polymeric stents makes the response tostress less predictable than in metallic stents. Creep can cause adeployed stent to retract or recoil radially inward, reducing theeffectiveness of a stent in maintaining desired vascular patency.

To address these and other problems, the mechanical properties of apolymer can be modified through various processing techniques, such as,by applying stress to a polymer. The application of stress can inducemolecular orientation along the direction of stress which can modifymechanical properties along the direction of applied stress. Forexample, strength and modulus are some of the important properties thatdepend upon orientation of polymer chains in a polymer. Molecularorientation refers to the relative orientation of polymer chains along alongitudinal or covalent axis of the polymer chains.

A polymer may be completely amorphous, partially crystalline, or almostcompletely crystalline. A partially crystalline polymer includescrystalline regions separated by amorphous regions. The crystallineregions do not necessarily have the same or similar orientation ofpolymer chains. However, a high degree of orientation of crystallitesmay be induced by applying stress to a semi-crystalline polymer. Thestress may also induce orientation in the amorphous regions. An orientedamorphous region also tends to have high strength and high modulus alongan axis of alignment of polymer chains. Additionally, for some polymersunder some conditions, induced alignment in an amorphous polymer may beaccompanied by crystallization of the amorphous polymer into an orderedstructure. This is known as stress induced crystallization.

As indicated above, due to the magnitude and directions of stressesimposed on a stent during use, it is important for the mechanicalstability of the stent to have suitable mechanical properties, such asstrength and modulus, in the axial and circumferential directions.Therefore, it can be advantageous to modify the mechanical properties ofa polymeric tube or sheet substrate, to be used in the fabrication of astent pattern, by induced orientation from applied stress in the axialdirection, circumferential direction, or both. Since highly orientedregions in polymers tend to be associated with higher strength andmodulus, it may be desirable to incorporate processes that inducealignment of polymer chains along one or more preferred axes ordirections into fabrication of stents.

The degree of radial expansion, and thus induced circumferentialorientation and radial strength, of a tube can be quantified by a radialexpansion ratio:

RE=(Inside Diameter of Expanded Tube, ID_(E))/(Original Inside Diameterof Tube, ID_(O))

The RE ratio can also be expressed as a percent expansion:

% RE=(RE−1)×100%

In some embodiments, a stent substrate in the form of a polymeric tubemay be deformed by blow molding. In blow molding, the tube can beradially deformed or expanded by increasing a pressure in the tube byconveying a fluid into the tube. The fluid may be a gas, such as air,nitrogen, oxygen, or argon. The polymer tube may be deformed or extendedaxially by applying a tensile force by a tension source at one end whileholding the other end stationary. Alternatively, a tensile force may beapplied at both ends of the tube. The tube may be axially extendedbefore, during, and/or after radial expansion.

In some embodiments, blow molding may include first positioning a tubein a tubular mold. The mold may act to control the degree of radialdeformation of the tube by limiting the deformation of the outsidediameter or surface of the tube to the inside diameter of the mold. Theinside diameter of the mold may correspond to a diameter less than orequal to a desired diameter of the polymer tube. Alternatively, thefluid temperature and pressure may be used to control the degree ofradial deformation by limiting deformation of the inside diameter of thetube as an alternative to or in combination with using the mold. Thetemperature of the tube can be heated to temperatures above the Tg ofthe polymer during deformation to facilitate deformation. The polymertube may also be heated prior to, during, and subsequent to thedeformation.

Properties of a polymer such as fracture toughness are affected by theoverall degree of crystallinity and the number and size of crystaldomains in a semi-crystalline polymer. It has been observed thatfracture toughness is increased by having a large number of smallcrystal domains in a polymer surrounded by an amorphous domain. Such acrystal structure can also reduce or prevent creep.

FIG. 2 shows a method of manufacturing stents in accordance with anembodiment of the present invention. The method comprises increasing 200the radial strength of the stent substrate in order to eliminate orreduce inward recoil of a stent manufactured from the substrate. Thestent substrate can be a polymeric tube or sheet. Increasing 200 theradial strength may include radially expanding 210 the stent substrate.Radial expansion 210 may induce 220 polymer chains in individual stentstruts later formed from the substrate to have a preferentialorientation in a circumferential direction as compared to an axialdirection. The axial direction or orientation corresponds to the overalllengthwise direction of the stent as represented by axis 160 in FIG. 1and line A-A in FIGS. 3 and 4. The circumferential direction ororientation corresponds to the direction along the circumference of thestent substrate as represented by line B-B in FIGS. 3 and 4 and circle728 in FIG. 5.

Radial expansion 210 may be achieved by blow molding a stent substratethat is in the form of a polymer tube. Prior to radial expansion 210,the tube has an original inner diameter of ID_(O). After radialexpansion 210, the expanded tube has an inner diameter of ID_(E). Insome embodiments, radial expansion 210 is performed so that the percentradial expansion % RE (equal to (ID_(E)/ID_(O)−1)×100%) is between about300% and 400%, which corresponds to ID_(E) between about four timesID_(O) and about five times ID_(O). In other embodiments, % RE isbetween about 400% and 500%, which corresponds to ID_(E) between aboutfive times ID_(O) and about six times ID_(O). In yet other embodiments,radial expansion 210 is performed until % RE is about 500%, or greater,which corresponds to ID_(E) that about six times ID_(O), or greater.

Polymer chains in a stent substrate may initially have a preferentialorientation in the axial direction as a result of extrusion, injectionmolding, tensile loading, machining, or other process used to form thestent substrate. In some embodiments, radial expansion 210 of a stentsubstrate having polymer chains with an initial axial orientation willreorient or induce 220 the polymer chains to have a circumferentialorientation. In other embodiments, radial expansion 210 of a stentsubstrate having polymers with an initial axial orientation may induce230 polymer molecule chains to have a biaxial orientation. In a biaxialorientation, the polymer chains are oriented in a direction that isneither preferentially circumferential nor preferentially axial. In thisway, the polymer chains can be oriented in a direction substantiallyparallel to the lengthwise axis of individual stent struts so as toincrease the overall radial strength of the stent.

The words “substantially” or “substantial” as used herein to modify acondition means that the condition is present in absolute or perfectform, as well as in a form that is not necessarily absolute or perfectbut would be considered close enough to those of ordinary skill in theart to warrant designating the condition as still being present. Forexample, “substantially parallel” encompasses perfectly parallel as wellas not perfectly parallel but parallel enough to those of ordinary skillin the art to warrant designating the parallel condition as beingpresent.

The method also includes making 240 a stent pattern from the substrateafter inducing polymer chains to have a particular preferentialorientation. Making 240 the stent pattern may include removing 250portions of the substrate by laser cutting and/or chemical etching,leaving only stent struts, bending elements, and other necessarystructures. The desired strength, toughness, and/or flexibility ofindividual structural elements in the stent pattern can achieved byforming such elements substantially parallel with the orientation of thepolymer molecule chains. The stent pattern can be as shown in FIGS. 3and 4 or variations of FIGS. 3 and 4.

Optionally, after making 240 the stent pattern, the stent may be crimped260 onto a balloon catheter or other stent delivery device. Prior to orduring crimping 260, the stent may be heated to a crimping temperatureTc. In some embodiments, Tc is greater than ambient room temperature Tato minimize or prevent outward recoil of the stent to a larger diameterafter crimping. Outward recoil undesirably increases the deliveryprofile of the stent and may cause the stent to prematurely detach fromthe catheter during delivery to a target treatment site within a vessel.Also, Tc is preferably below Tg to reduce or eliminate stress relaxationduring crimping. Stress relaxation during or after crimping leads to agreater probability of cracking during subsequent deployment of thestent. To reduce or prevent such cracking, the difference between Tc andTg can be maximized by increasing Tg through stress inducedcrystallization.

After manufacturing, the stent can be deployed 270 inside a blood vesselfrom a crimped diameter to a deployed outer diameter OD_(D). In someembodiments, OD_(D) is greater than OD_(E), the outer diameter as aresult of radial expansion of the stent substrate. Preferably, OD_(D) isselected so that no cracks are formed in the stent during deployment. Insome embodiments, OD_(D) is 3.5 mm (0.1378 in). In other embodiments,OD_(D) is 4.0 mm (0.1575 in).

If the stent was crimped 260 onto a balloon catheter, the deployment 270of the stent can include inflating the balloon catheter to urge thestent to move from its crimped configuration to an expanded, deployedconfiguration. In other embodiments, the stent may be self-expanding anddeployment 270 of the stent can include removing a sheath or otherconstraining device from around the stent to allow the stent toself-expand.

It will be appreciated that the method of FIG. 2 is applicable to manytypes of bodily lumens or organs. Examples of such organs include, butare not limited to, vascular organs such as, for example, coronaryarteries or hepatic veins; renal organs such as, for example, urethrasand ureters; biliary organs such as, for example, biliary ducts;pulmonary organs such as, for example, tracheas, bronchi andbronchioles; and gastrointestinal organs such as, for example, esophagiand colons.

FIG. 3 depicts an exemplary stent pattern 300 cut from a polymericsubstrate. Stent pattern 300 is shown in a flattened condition so thatthe pattern can be clearly viewed. When the stent pattern 300 is in acylindrical form, it forms a radially expandable stent. Stent pattern300 includes a plurality of cylindrical rings 305 with each ringincluding a plurality of diamond shaped cells 310. Embodiments of stentpattern 300 may have any number of rings 305 depending on a desiredlength of a stent. For reference, line A-A extends in an longitudinal oraxial direction, which is the same direction of axis 160 in FIG. 1.Diamond shaped cells 310 include bending elements 315 and 320. Stentpattern 300 can also includes bending elements 325 and 330. The anglesof bending elements 315, 320, 325, and 330 correspond to angles θ₁, θ₂,θ₃, and θ₄. Angles θ₁, θ₂, θ₃, and θ₄ are equivalent to or about 42, 42,41, and 21 degrees, respectively. In other embodiments, angles θ₁, θ₂,θ₃ are about 24 degrees to about 29 degrees, and angle θ₄ is about 12degrees to about 15 degrees. Diamond shaped cells 310 are made up of bararms 335 and 340 that form bending element 315 and bar arms 345 and 350that form bending element 320.

When stent 300 is crimped, bending elements 315, 320, 325, and 330 flexinward and angles θ₁, θ₂, θ₃, and θ₄ decrease, allowing the stent to beradially compressed. With respect to bending elements 315, 320, and 325,struts on either side of the bending elements bend toward each other.However, in bending element 330, the strut of the diamond-shaped elementtends to bend toward a linking arm 355, which tends to remain relativelyparallel to the longitudinal axis during crimping.

Pattern 300 includes linking arms 355 that connect adjacent cylindricalrings. Linking arms 355 are parallel to line A-A and connect adjacentrings between intersection 360 of circumferentially adjacentdiamond-shaped elements 310 of one ring and intersection 360 ofcircumferentially adjacent diamond shaped elements 310 of an adjacentring. As shown, linking elements connect every other intersection alongthe circumference.

The curved portions of bending elements experience substantial stressand strain when a stent is crimped and deployed. Therefore high strengthand toughness are very important in these regions. Ideally, the mosteffective polymer chain orientation to improve fracture toughness isalong the length of the axis of the strut. Radial expansion impartsorientation and fracture toughness along the circumferential direction,as shown by line B-B.

FIG. 4 shows another stent pattern 700 in accordance with an embodimentof the present invention. The stent pattern 700 includes various struts702 oriented in different directions and gaps 703 between the struts.Each gap 703 and the struts 702 immediately surrounding the gap 703defines a closed cell 704. At the proximal and distal ends of the stent,a strut 706 includes depressions, blind holes, or through holes adaptedto hold a radiopaque marker that allows the position of the stent insideof a patient to be determined.

One of the cells 704 is shown with cross-hatch lines to illustrate theshape and size of the cells. In the illustrated embodiment, all thecells 704 have the same size and shape. In other embodiments, the cells704 may vary in shape and size.

The stent pattern 700 is shown in a planar or flattened view for ease ofillustration and clarity, although the stent pattern 700 on a stentactually extends around the stent so that line A-A is substantiallyparallel to the central axis of the stent. The pattern 700 isillustrated with a bottom edge 708 and a top edge 710. On a stent, thebottom edge 708 meets the top edge 710 so that line B-B forms a circlearound the stent. In this way, the stent pattern 700 forms sinusoidalhoops or rings 712 that include a group of struts arrangedcircumferentially. The rings 712 include a series of crests 707 andtroughs 709 that alternate with each other. The sinusoidal variation ofthe rings 712 occurs primarily in the axial direction, not in the radialdirection. That is, all points on the outer surface of each ring 712 areat substantially the same radial distance away from the central axis ofthe stent.

Still referring to FIG. 4, the rings 712 are connected to each other byanother group of struts that have individual lengthwise axes 713substantially parallel to line A-A. The rings 712 are capable of beingcollapsed to a smaller diameter during crimping and expanded to theiroriginal diameter or to a larger diameter during deployment in a vessel.

In other embodiments, the stent may have a different number of rings 712and cells 704 than what is shown in FIG. 4. The number of rings 712 andcells 704 may vary depending on the desired axial length and deployeddiameter of the stent. For example, a diseased segment of a vessel maybe relatively small so a stent having a fewer number of rings can beused to treat the diseased segment.

FIG. 5 shows a simplified diagram of the stent pattern 700 in the formof a cylindrical tube. The sinusoidal variation of the rings 712 and thestruts linking the rings to each other are omitted for clarity. Therings 712 have center points 720. At least two of the center points 720define the central axis 724 of the stent. The central axis 724 and lineA-A in FIGS. 4 and 6 are substantially parallel to each other. The ringshave an abluminal surface 692 and a luminal surface 694. The abluminalsurface 692 faces outward and normally contacts the wall of theanatomical lumen in which the stent is deployed. The luminal surface 694faces inward toward the center of the lumen when deployed.

FIG. 6 shows a detailed view of a portion 716 of the stent pattern 700of FIG. 4. The rings 712 include linear ring struts 730 and curved hingeelements 732. The ring struts 730 are connected to each other by thehinge elements 732. In some embodiments, the ring struts and hingeelements are formed from a polymeric substrate that was radiallyexpanded in the circumferential direction represented by a dotted circle728 in FIG. 5 and line B-B in FIGS. 4 and 6.

Radial expansion of the substrate used to form the stent pattern 700 ispreferably between about 300% and about 700%, which corresponds toID_(E) that is between about four to about eight times ID_(O). In someembodiments, radial expansion is between about 400% and about 600%,which corresponds to ID_(E) that is between about five to about seventimes ID_(O). In other embodiments, radial expansion is at or about500%, which corresponds to ID_(E) that is at or about six times ID_(O).

The hinge elements 732 are adapted to flex, which allows the rings 712to move from a non-deformed configuration to a deformed configuration.As used herein, “non-deformed configuration” refers to the state of therings prior to being crimped to a smaller diameter for delivery throughan anatomical lumen. For example, in embodiments in which a stent isformed by laser cutting a radially expanded polymer tube, thenon-deformed configuration is the state of the rings after radialexpansion of the polymer tube and laser cutting of the polymer tube toform the rings. As used herein, “deformed configuration” refers to thestate of the rings upon some type of deformation, such as crimping ordeployment.

FIGS. 3-6 show the rings 712 in the non-deformed configuration.Referring to FIG. 5, the rings 712 have an initial outer diameter 729when in the non-deformed configuration. In some embodiments, the initialouter diameter 729 of the rings 712 is substantially equivalent toOD_(E), the outer diameter of the stent substrate after the stentsubstrate is radially expanded (outer diameter of expanded tube).

Referring again to FIG. 6, line B-B lies on a reference planeperpendicular to the central axis 724 (FIG. 5). When the rings 712 arein the non-deformed configuration, as shown in FIG. 6, each ring strut730 is oriented at a non-zero angle C relative to the reference plane.The non-zero angle C is less than 40 degrees in the illustratedembodiment. Preferably, the non-zero angle C is less than 35 degrees,and more narrowly the angle C is between about 25 degrees and about 28degrees. In other embodiments, the angle C can have other values.

Also, the ring struts 730 are oriented at an interior angle D relativeto each other. The interior angle D is greater than 100 degrees in theillustrated embodiment. Preferably, the interior angle D is greater than110 degrees, and more narrowly, the angle D is between about 124 degreesand about 130 degrees. In other embodiments, the interior angle D canhave other values.

Referring once again to FIG. 6, the stent also includes link struts 734connecting the rings 712 together. The link struts 734 are orientedsubstantially parallel to line A-A and the central axis 724 (FIG. 5).The ring struts 730, hinge elements 732, and link struts 734 define aplurality of W-shaped closed cells 736. The boundary or perimeter of oneW-shaped cell 736 is darkened in FIG. 6 for clarity. The W-shapes appearrotated 90 degrees counterclockwise. Each of the W-shaped cells 736 isimmediately surrounded by six other W-shaped cells 736, meaning that theperimeter of each W-shaped cell 736 merges with a portion of theperimeter of six other W-shaped cells 736. Stated another way, eachW-shaped cell 736 abuts or touches six other W-shaped cells 736.

The perimeter of each W-shaped cell 736 includes eight of the ringstruts 730, two of the link struts 734, and ten of the hinge elements732. Four of the eight ring struts form a proximal side of the cellperimeter and the other four ring struts form a distal side of the cellperimeter. The opposing ring struts on the proximal and distal sides aresubstantially parallel to each other.

Within each of the hinge elements 732 there is an intersection point 738toward which the ring struts 730 and link struts 734 converge. There isan intersection point 738 adjacent each end of the ring struts 730 andlink struts 734. Distances 740 between the intersection points adjacentthe ends of rings struts 730 are substantially the same for each ringstrut 730. In other embodiments, such as shown in FIG. 10, some of thering struts 730 may be longer than other ring struts 730 so thatdistances 740 may vary. For example, distances 740 may vary to allow fora variation in mechanical characteristics at different portions of thestent.

Referring again to FIG. 6, distances 742 between the intersection pointsadjacent the ends of horizontal link struts 734 are substantially thesame for each link strut 734. In other embodiments, such as shown inFIG. 11, some of the link struts 734 may be longer than other linkstruts 734 so that distances 742 may vary to allow for a variation inmechanical characteristics at different portions of the stent.

Also, distances 740 are substantially the same as distances 742. Inother embodiments, distances 740 and 742 are different from each otherto allow for a variation in mechanical characteristics at differentportions of the stent.

The ring struts 730 have widths 737 that are uniform along theindividual lengthwise axis 713 of the ring strut. The link struts 734have widths 739 that are also uniform along the individual lengthwiseaxis 713 of the link strut.

As shown in FIG. 6, the interior space of each W-shaped cell 736 has anaxial dimension 744 parallel to line A-A and a circumferential dimension746 parallel to line B-B. The axial dimension 744 is substantiallyconstant with respect to circumferential position. That is, axialdimensions 744A adjacent the top and bottom ends of the cells 736 aresubstantially the same as axial dimensions 744B further away from theends. The constant axial dimension 744 provides an improved strutdistribution compared to the stent pattern of FIG. 3.

In the illustrated embodiment of FIG. 6, axial and circumferentialdimensions 744 and 746 are the same among the W-shaped cells 736. Inother embodiments, the axial dimension 744 and/or circumferentialdimension 746 may differ among cells 736 to allow for a variation inmechanical characteristics at different portions of the stent.

FIGS. 7 and 8 show a portion of one ring 712 in two different deformedconfigurations. In FIG. 7, the ring 712 has been radially compressed toa diameter less than its initial outer diameter 729 (FIG. 5), such aswhen the stent is crimped onto a catheter. During such compression, thering struts 730 pivot about the hinge elements 732 so that the ringstruts 730 fold toward each other and become oriented at an angle Erelative to the reference plane represented by line B-B. Angle E isgreater than corresponding angle C in FIG. 6 which shows rings 712 inthe non-deformed configuration. Also, the ring struts 730 are orientedat an interior angle F relative to each other. Angle F is less thancorresponding angle D in FIG. 6.

In FIG. 8, the ring 712 has been radially expanded, after manufacturing,to a deployed configuration. Radial expansion of the rings 712 is not tobe confused with radial expansion of the stent substrate duringmanufacturing. When the ring 712 is radially expanded, ring struts 730pivot about the hinge elements 732 and the ring struts 730 becomeoriented at an angle G relative to the reference plane represented byline B-B. When the ring 712 is expanded to a diameter greater than itsnon-deformed initial diameter 729 (FIG. 5), angle G is less thancorresponding angle C in FIG. 6. Also, the ring struts 730 are orientedat an interior angle H relative to each other. Angle H is greater thancorresponding angle D in FIG. 6.

A test was performed to study long term lumen patency using differentstent patterns formed from polymer tube substrates that were radiallyexpanded during manufacturing. Case 1 corresponds to a stent having thestent pattern of FIG. 2 cut from a polymer tube substrate that waspreviously radially expanded 300% to a diameter of 2.13 mm (0.084inches). Case 2 corresponds to a stent having the stent pattern of FIGS.4 and 6 cut from a polymer tube substrate that was previously radiallyexpanded 500% to a diameter of 3.48 mm (0.137 inches). In both cases,the polymer tube was an extruded tube of poly(L-lactide), a bioabsorablepolymer. FIG. 9 is a perspective view of a portion of the stent of Case2.

In Case 1, ring struts 335, 340, 345, and 350, when in the non-deformedconfiguration, were at about 75 degrees to about 78 degrees relative tothe circumferential direction, that is the direction of radialexpansion. Also, ring struts 335 and 345 were oriented relative to ringstruts 340 and 350 at interior angles (θ₁ and θ₃ in FIG. 3) of about 24degrees to about 29 degrees.

In Case 2, ring struts 730, when in the non-deformed configuration, wereat 25 degrees to 28 degrees from the circumferential direction. Also,ring struts 730 were oriented relative to each other at an interiorangle (angle D in FIG. 6) of about 124 degrees to about 130 degrees.

Compared to Case 1, Case 2 exhibited less inward recoil (decrease indiameter) after deployment, resulting in larger lumens and better stentapposition or contact with surrounding tissue. The results of the testindicate that 500% radial expansion combined with struts arranged in thepattern of FIGS. 4 and 6 (Case 2) provided greater radial strength than300% radial expansion combined with struts arranged in the pattern ofFIG. 3 (Case 1). The results were unexpected in that excessive radialexpansion of the substrate is known to cause an increase in strutfractures during crimping or upon deployment, and that excessively largeinterior angles are known to prevent stents from crimping easily and ina controlled manner.

It is believed that the polymer chains in Case 1 do not readily line upwith the rings as compared to Case 2, leading to a stent that is lessresistant to mechanical creep deformation as compared to Case 2. With500% radial expansion in Case 2, the polymer chains are substantiallycircumferentially oriented. The interior angles between stent struts inCase 2 allow the stent struts to line up well with the circumferentiallyoriented polymer chains. It is also believed that smaller interiorangles than those in Case 2 would increase the likelihood of fracturessince expansion loads would be applied against the “grain” of thesubstrate, that is, against the circumferential orientation of thepolymer chains.

In other tests, stents were made from tubes of poly(L-lactide) substratethat were radially expanded to different levels, namely 400%, 500%,600%, and 700%. The stents were crimped at different crimpingtemperatures, deployed, then inspected for fractures. At a crimpingtemperature of about 50° C., the group of stents made from 500% radiallyexpanded tubes had the lowest average number of fractures. At crimpingtemperatures of about 30° C., 50° C., and 60° C., the group of stentsmade from 700% radially expanded tubes had the highest average number offractures.

Non-Polymeric Materials

It will be appreciated that the above disclosed stent patterns may beapplied to non-polymer stent substrates as well. A non-polymer substrateof a stent may be made of a metallic material or an alloy such as, butnot limited to, cobalt chromium alloy (ELGILOY), stainless steel (316L),high nitrogen stainless steel, e.g., BIODUR 108, cobalt chrome alloyL-605, “MP35N,” “MP20N,” ELASTINITE (Nitinol), tantalum, nickel-titaniumalloy, platinum-iridium alloy, gold, magnesium, or combinations thereof“MP35N” and “MP20N” are trade names for alloys of cobalt, nickel,chromium and molybdenum available from Standard Press Steel Co.,Jenkintown, Pa. “MP35N” consists of 35% cobalt, 35% nickel, 20%chromium, and 10% molybdenum. “MP20N” consists of 50% cobalt, 20%nickel, 20% chromium, and 10% molybdenum.

Hinge Element Details

FIG. 12 shows a detailed view of linear ring struts 730, hinge elements732, and linear link struts 734 of the pattern shown in FIGS. 4 and 6.FIG. 12 corresponds to the boxed area 799 in FIG. 6, and FIG. 6corresponds to the boxed area 716 in FIG. 4. In FIG. 12, every hingeelement 732 is directly connected to exactly two linear ring struts 730.There are two types of hinge elements 732: U-shaped hinge elements 732a, and Y-shaped hinge elements 732 b, 732 c. A hinge element is U-shapedwhen it has exactly two ends and is Y-shaped when it has exactly threeends. The U-shaped hinge elements 732 a connect two linear ring struts730 together, exclusively. The Y-shaped hinge elements 732 b, 732 cconnect two linear ring struts 730 and a linear link strut 734 together,exclusively. There is a Y-shaped hinge element at each end of everylinear link strut 734.

The U-shaped hinge elements 732 a are not tangent to the linear ringstruts 730. The non-tangent relationship can be seen from referencelines 800 (illustrated as broken lines) which coincide with and extendout from the straight edges of linear ring struts 730. The edges of thestruts are illustrated as solid lines. The U-shaped hinge elements 732 abow out from the linear ring struts 730 in such a way that there areclearly noticeable interior spaces 802 between the reference lines 800and the edges of the U-shaped hinge elements 732 a which cross over thereference lines 800.

Still referring to FIG. 12, each U-shaped hinge element 732 a includes amiddle segment 804 and two end segments 806. Broken lines areillustrated to show boundaries between the middle and end segments. Eachmiddle segment 804 has edges that are curved continuously throughout.The end segments 806 have edges that are straight throughout. The endsegments 806 provide a geometric transition from the linear ring struts730 to the middle segment 804 of the U-shaped hinge element 732 a.

Due to its non-tangency, the U-shaped hinge elements 732 a have morematerial than would otherwise be possible for a given bend radius.Stated differently, a non-tangent U-shaped hinge element 732 a of agiven bend radius, as shown in FIG. 12, has more material than asimilarly located tangent U-shaped hinge element of the same bendradius. A way to measure the amount of material is to measure the lengthof the inside curve 808 of the U-shaped hinge elements 732 a. The insidecurve of one U-shaped hinge element 732 a is illustrated as a thick lineto distinguish it from other features of the stent pattern.

This additional material can help reduce cracks at the hinge elementswhen a stent is expanded from a crimped state to a fully deployed state,especially when the stent at a fully deployed state (e.g., block 270 inFIG. 2) is greater in diameter than the stent prior to crimping (e.g.,block 250 in FIG. 2). As previously mentioned, a stent can be formed byradially expanding a stock tube to make a precursor tube having aselected outer diameter OD_(E), then a stent pattern is cut onto theprecursor tube (e.g., block 250 in FIG. 2), followed by crimping to asmaller diameter and subsequent radial expansion to a deployed diameterOD_(D) (e.g., block 270 in FIG. 2). For various reasons, it may bedesirable to over-expand the stent so that OD_(D) is greater thanOD_(E), such to address a problem with post-deployment recoil of thesent. The additional material provided by the non-tangent hinge elements732 a allows the stent to expand to OD_(D) greater than OD_(E) with alower incidence of cracks. Also, stresses in the non-tangent hingeelements 732 a during stent deployment are distributed over a greateramount of material, which may also reduce the incidence of cracks.

Referring to FIG. 12, the linear link struts 730 have overall lengths L,which can be measured as the length of one of the straight edges of thestrut. Also, the linear ring struts 730 are oriented relative to thelinear link struts 734 at the same angles. Each linear link strut 734 isconnected to a first ring strut 730 and a second ring strut 730. Thefirst and second ring struts are connected to opposite ends of thelinear link strut. The first ring strut is oriented at an angle Jrelative to the link strut. The second ring strut is oriented at anangle K relative to the link strut. Angles J and K are substantially thesame. Also, the linear ring struts 730 and the U-shaped hinge elements732 a have a width 737 that is substantially uniform throughout thelinear ring struts and U-shaped hinge elements. The width is generallyaxial (as opposed to radial) in direction and is measured normal orperpendicular to an edge of the hinge element or ring strut.

FIG. 13 shows another embodiment of a stent pattern having repeatingW-shaped closed cells similar to FIGS. 4 and 6 in that the cells arearranged in an offset brick pattern. There are linear ring struts 730′,hinge elements 732′, and linear link struts 734′. There are U-shapedhinge elements 732 a′ located circumferentially between the linear linkstruts 734′, and Y-shaped hinge elements 732 b′, 732 c′ located at theends of the linear link struts 734′. Broken lines are illustrated toshow boundaries of the hinge elements 732 a′, 732 b′, 732 c′. The edgesof the struts and hinge elements are illustrated as solid lines.

A difference from the embodiment of FIG. 12 is that in FIG. 13 theU-shaped hinge elements 732 a′ are tangent to the linear ring struts730′. The tangent relationship is evident from the absence of interiorspaces between reference lines 800′ and the edges of the U-shaped hingeelements 732 a′. The reference lines 800′ coincide with and extend outfrom the edges of the linear ring struts 730′. Notice that the edges ofthe U-shaped hinge elements 732 a′ do not cross over the reference lines800′, so that an interior space (similar to space 800 in FIG. 12) is notformed. In addition, the bend radii of the tangent U-shaped hingeelements 732 a′ of FIG. 13 are smaller than those for the non-tangenthinge elements 732 a of FIG. 12. Further, the overall length of linearring struts 730′ is longer in FIG. 13 than linear ring struts 730 inFIG. 12. The combination of these features makes the U-shaped hingeelements 732 a′ of FIG. 13 have less material than those in FIG. 12. Thedecrease in material is evident from the fact that the length of theinside curve 808′ of the tangent U-shaped hinge element 732 a′ is lessthan the inside curve 808 of the non-tangent U-shaped hinge element 732a of FIG. 12. One advantage to having less material inside curve 808′ ofthe tangent U shaped hinge element 732 a′ is that the elements undergo alower amount of compressive deformation when crimped, thereby preventingdamage. This damage may not be readily apparent immediately aftercrimping yet lead to strut fractures when the stent is deployed later.

Another difference from FIG. 12 is that for each W-shaped closed cell ofthe stent pattern, the crests at the U-shaped hinge elements 732 a′adjacent a linear link strut 734 are circumferentially offset from eachother. The circumferential position of the crests are indicated bydash-dot reference lines, and the circumferential offset distance isindicated by the letter “d”.

Referring to FIG. 13, the linear link struts 730′ have overall lengthsL′, which can be measured as the length of one of the straight edges ofthe strut. Also, the linear ring struts 730′ are oriented relative tothe linear link struts 734′ at the different angles. Each linear linkstrut 734′ is connected to a first ring strut 730′ and a second ringstrut 730′. The first and second ring struts are connected to oppositeends of the linear link strut. The first ring strut is oriented at anangle J′ relative to the link strut. The second ring strut is orientedat an angle K′ relative to the link strut. Angle J′ is greater thanangle K′. Angle J′ is less than both angles J and K of FIG. 12. Also,the linear ring struts 730′ and the U-shaped hinge elements 732 a′ havea width 737′ that is substantially uniform throughout the linear ringstruts and U-shaped hinge elements.

A change in angles J′ and K′ could affect the mechanical properties ofthe link struts and hinge elements. This is because blow molding andradial expansion of the stock tube creates a precursor tube with polymerchains having a preferred orientation at the molecular level. Structuralelements of the stent which are cut from the precursor tube are expectedto be more rigid along the direction of the polymer chain orientation.For example, linear ring struts that are cut at an angle thatsubstantially coincides with the polymer chain orientation may providegreater hoop strength and greater resistance to radial loads afterdeployment, but may make the stent too stiff in the longitudinaldirection after crimping. Longitudinal flexibility is desired tofacilitate delivery of a stent through tortuous vasculature of apatient. Linear ring struts that are cut at an angle offset from thepolymer chain orientation may be more flexible and better able towithstand mechanical stress from crimping and deployment. Also, theconfiguration and orientation of hinge elements relative to the polymerchain orientation may affect post-crimp and post-deployment recoilcharacteristics of the stent.

Example 1

Stock tubing made of extruded PLLA was radially expanded in a blow moldto form a precursor tube having an outer diameter of about 2.5 mm (about0.100 inches). The precursor tube was laser cut to have the stentpattern of FIGS. 4 and 6 except that the U-shaped hinge elements weretangent to the linear ring struts. The resulting stent was then crimpedto an outer diameter of 1.3 mm onto a balloon catheter, then deployed byradially expanding to an outer diameter of 3.8 mm.

Example 2

Stock tubing made of extruded PLLA was radially expanded in a blow moldto form a precursor tube having an outer diameter of about 2.5 mm. Theprecursor tube was laser cut to have the stent pattern of FIGS. 4 and 6with the non-tangent U-shaped hinge elements of FIG. 12 in order toincrease the amount of material between the linear ring struts. Theresulting stent was then crimped to an outer diameter of 1.3 mm onto aballoon catheter, then deployed by radially expanding to an outerdiameter of 3.8 mm. Compared to Example 1, Example 2 exhibited lesscracks at bent areas between the linear ring struts as a result ofcrimping and deployment.

Example 3 and FIGS. 12 & 14

Stock tubing made of extruded PLLA was radially expanded in a blow moldto form a precursor tube having an outer diameter of 3.5 mm. Theprecursor tube was laser cut to have the stent pattern of FIGS. 4 and 6with the non-tangent U-shaped hinge elements of FIG. 12. The resultingstent was then crimped to an outer diameter of 1.3 mm onto a ballooncatheter, then deployed by radially expanding to an outer diameter of3.8 mm.

In FIG. 12, the following dimensions apply prior to crimping anddeployment. Each of the U-shaped hinge elements and the linear ringstruts have both a uniform width of about 0.17 mm±0.04 mm and uniformthickness of about 0.15 mm±0.03 mm. The thickness is the same as theradial thickness of the blow molded precursor tube and can be measuredas the difference between the inner and outer diameters of the precursortube. Each linear ring strut 730 is oriented relative to the linear linkstruts 734 at an angle (J and K) of approximately 65 degrees. The middlesegment 804 of each U-shaped hinge element has an inner edge radius Riand outer edge radius Ro. The inner edge radius Ri corresponds to theinner edge and is approximately 0.33 mm. The outer edge radius Rocorresponds to the outer edge and is approximately 0.48 mm.

A photograph of the stent after deployment is shown in FIG. 14. TheU-shaped and Y-shaped hinge elements have damage clearly visible ascrazing and cracks. The visible damage extends at least one third intothe width 737 of the U-shaped and Y-shaped hinge elements.

Example 4 and FIGS. 13 & 15

Stock tubing made of extruded PLLA was radially expanded in a blow moldto form a precursor tube having an outer diameter of 3.5 mm, as inExample 3. The precursor tube was laser cut to have the stent pattern ofFIGS. 4 and 6 with the tangent U-shaped hinge elements of FIG. 13. Theresulting stent was then crimped to an outer diameter of 1.3 mm onto aballoon catheter, then deployed by radially expanding to an outerdiameter of 3.8 mm.

In FIG. 13, the following dimensions apply prior to crimping anddeployment. Each of the U-shaped hinge elements and the linear ringstruts have both a uniform width of about 0.17 mm±0.04 mm and uniformthickness of about 0.15 mm±0.03 mm, which are substantially the same asin Example 3. Each U-shaped hinge element has an inner edge radius Ri′and outer edge radius Ro′, which are smaller than corresponding radii inExample 3. The inner edge radius Ri′ corresponds to the inner edge andis approximately 0.25 mm. The outer edge radius Ro′ corresponds to theouter edge and is approximately 0.41 mm.

The linear ring struts are oriented relative to the linear link strutsat different angles. Opposite ends of each linear link strut arerespectively connected to a first ring strut and a second ring strut.The first and second ring struts are connected to opposite ends of thelinear link strut. The first ring strut is oriented relative to the linkstrut at an angle (J′) of approximately 50 degrees. The second ringstrut is oriented relative to the link strut at an angle (K′) ofapproximately 40 degrees. By comparison, angles J and K areapproximately 65 degrees for Example 3 (FIG. 12).

A photograph of the stent after deployment is shown in FIG. 15. Comparedto Example 3, Example 4 exhibited a significant decrease in crazing andcracks at bent areas between the linear ring struts 730′. No substantialdamage was observed on the hinge elements. The improvement in crackresistance was unexpected in that the tangent U-shaped hinge elementsprovided less material for distributing stresses as compared tonon-tangent U-shaped hinge elements of Example 3.

A summary of Examples 1 through 4 is shown in TABLE 1. It is to beunderstood that the stents for Examples 1 through 4 were made of PLLAand had strut patterns defined by W-shaped closed cells in an offsetbrick arrangement, with some differences being in the parameters listedin TABLE 1. In Examples 1 through 4, the strut pattern included pairs ofrings made up of exactly three W-shaped closed cells arrangedcircumferentially. The circumferential series of three W-shaped closedcells repeat in the axial direction to form the tubular body of thestent, wherein none of the linear link struts are directly connected toa linear link strut of an axially adjacent cell. Also, all expandedprecursor tubes have the same thickness in Examples 1 through 4.

TABLE 1 Parameter Example 1 Example 2 Example 3 Example 4 Outer 2.5 mm2.5 mm 3.5 mm 3.5 mm Diameter of Precursor Tube Outer 1.3 mm 1.3 mm 1.3mm 1.3 mm Diameter after Crimping Outer 3.8 mm 3.8 mm 3.8 mm 3.8 mmDiameter after Deployment U-shaped tangent not tangent not tangenttangent Hinge: Tangency U-shaped 33 degrees 25 degrees Hinge: InnerRadius, Ri U-shaped 48 degrees 41 degrees Hinge: Outer Radius, Ro StrutAngle J = 65 J’ = 50 degrees degrees K = 65 K’ = 40 degrees degreesComparison Less damage Less post- Less damage than deployment thanExample 1 recoil than Example 3 Example 2

Curvilinear Length

Applicants have found that adjustment of another parameter, namelycurvilinear length, can be adjusted to increase the resistance ofpolymeric stents to fracture during deployment by balloons.

FIG. 16 shows another stent pattern 900 defined by W-shaped closed cellsin an offset brick arrangement, with U-shaped hinge elements tangent toadjacent linear ring struts. Test results for the pattern of FIG. 16will be discussed below in relation to the pattern of FIG. 4 which, aspreviously discussed, has U-shaped hinge elements that are not tangentto adjacent linear ring struts. It is to be understood that in FIGS. 4and 16, the top edge of the illustrated pattern wraps around and meetswith the bottom edge of the illustrated pattern when used to cut aprecursor tube.

In addition to the difference in tangency, the patterns of FIGS. 4 and16 differ in terms of curvilinear length which defines, in part, thesize of the W-shaped closed cells. FIG. 17 shows a detail view of arectangular region 910 of FIG. 16 bounded by broken lines, and FIG. 18shows a detailed view within the rectangular region 716 of FIG. 4. Thecurvilinear length of the W-shaped cell is the length of a curve 920″,920 that runs continuously through the center of the linear ring strutsand hinge elements at one side of cell. The curve runs from the Y-shapedhinge element 732 c″, 732 c at the top linear link strut 734″, 734 tothe Y-shaped hinge element 732 c″, 732 c at the bottom linear link strut734″, 734.

For the baseline pattern of FIGS. 4 and 18, the curvilinear length(i.e., the length of curve 920) is at or about 4.0 mm (0.159 inch).Also, the width of every linear link strut 734 in FIG. 18 is at or about0.13 mm (0.0050 inch).

For the modified pattern of FIGS. 16 and 17, the curvilinear length(i.e., the length of curve 920″) is at or about 4.5 mm (0.175 inch),which is significantly longer than that for the baseline pattern ofFIGS. 4 and 18. The increase in curvilinear length of the W-shaped cellswas accomplished by increasing crest radii, R″, and arc length. TheU-shaped hinge elements have a bend radius, R″, that is greater than inthe baseline pattern. Creating longer arcs is intended to reduce crazingand cracking by reducing stress concentrations during stent crimping anddeployment. Also, the width of every linear link strut 734″ in FIGS. 16and 17 is at or about 0.14 mm (0.0055 inch), which is thicker than thatfor the baseline pattern of FIGS. 4 and 18. The increase in width isintended to reduce risk of linear link strut fractures during bending.

First Comparative Test for Curvilinear Length

Samples of both the baseline pattern (FIGS. 4 and 18) and modifiedpattern (FIGS. 16 and 17) were tested to compare their resistance tofracture during deployment. The test procedure involved cutting thepattern onto a precursor tube of blow molded PLLA polymer, the tubehaving an outer diameter of about 3.5 mm (0.1378 inches) and an innerdiameter of about 3.2 mm (0.1260 inches). The stents were coated withdrug/polymer solution and the solvents in the solution were dried away.The stents were also sterilized. The stent was placed over a ballooncatheter, but not crimped in order to examine expansion capabilitywithout the effects from crimping. The balloon was inflated byincreasing pressure at increments of 1 atmosphere (atm). After initialinflation to 1 atm, the stent on the balloon was inspected forfractures. If no fractures were found, the inner diameter of the stent(outer diameter of the balloon) was determined and recorded. Theninflation pressure was increased to 2 atm (increase of 1 atm), and thestent was inspected again for fractures. If no fractures were found, theinner diameter of the stent (outer diameter of the balloon) was againdetermined and recorded. This process was repeated until the stentfractured.

The modified pattern of FIGS. 16 and 17 (having a longer curvilinearlength than the baseline pattern) exhibited greater resistance tofracture from overexpansion. On average, stents having the modifiedpattern with the longer curvilinear length (CL=4.5 mm) were expanded toan inner diameter of 4.48 mm before fracture occurred, as compared to3.84 mm for the baseline pattern (FIGS. 4 and 18) with the shortercurvilinear length (CL=4.0 mm).

Second Comparative Test for Curvilinear Length

Samples of both the baseline pattern (FIGS. 4 and 18) and modifiedpattern (FIGS. 16 and 17) were tested in the same manner as in the FirstComparative Test, except the stents were crimped onto the balloon priorto expansion by the balloon.

The modified pattern of FIGS. 16 and 17 (having a longer curvilinearlength than the baseline pattern) exhibited greater resistance tofracture from overexpansion. On average, stents having the modifiedpattern with the longer curvilinear length (4.5 mm) were expanded to aninner diameter of 4.54 mm before fracture occurred, as compared to 3.83mm for the baseline pattern with the shorter curvilinear length (4.0mm).

A summary of the first and second comparative tests is shown in TABLE 2.It is to be understood that the samples for the both tests used stentsconfigured to be crimped to an outer diameter of about 1.3 mm andexpanded to an outer diameter of about 3.8 mm. The stents were made of aPLLA precursor tube cut with strut patterns defined by W-shaped closedcells in an offset brick arrangement, as generally shown in FIGS. 4 and16. In both comparative tests, with and without crimping prior todeployment, the stent having greater CL was capable of being expanded toa greater diameter during deployment than the stent with smaller CL.

TABLE 2 CL = 4.0 mm CL = 4.5 mm (Baseline Pattern, Modified Pattern,FIGS. 4 & 18) FIGS. 16 & 17) Not Crimped Crimped Not Crimped Crimpedprior to prior to prior to prior to Deployment Deployment DeploymentDeployment Inner 3.84 mm 3.83 mm 4.48 mm 4.54 mm Diameter of Stent at 1atm Before Fracture

In some uses, stents having the baseline and modified patterns areexpanded during deployment to an inner diameter of about 3.5 mm. FIGS.19A and 19B show a stent with the baseline pattern (CL=4.0 mm, FIGS. 4and 18) in crimped and deployed states, respectively. FIG. 19B showsthat when the stent is deployed at 3.5 mm, the linear ring struts forminterior angles θ1 between each other. The interior angle is significantin that, if the angle is significantly decreased due to a designmodification, there is an increased risk that the stent will recoilafter deployment due to a tendency for the linear ring struts collapseback toward each other (i.e., the bending elements will tend to returnto the state that they were in after crimping).

FIGS. 20A and 20B show a stent with the modified pattern (CL=4.5 mm,FIGS. 16 and 17) in crimped and deployed states, respectively. FIG. 20Bshows that when the stent is deployed at 3.5 mm, the linear ring strutsform interior angles θ2 between each other. As can be seen from thefigures, the interior angle θ2 for the modified pattern appearssubstantially similar in size to the interior angle θ1 for the baselinepattern, with little noticeable decrease in interior angle. Thesimilarity of the interior angles indicates that even through thecurvilinear length was increased (to allow for significant fractureresistance from overexpansion), the risk of post-deployment recoil wasnot likely increased significantly.

FIGS. 21A and 21B show a stent with a further modified pattern havingCL=5.0 mm in crimped and deployed states, respectively. The “furthermodified” pattern is similar to the baseline pattern, except that thelength of the linear ring struts were increased so that the curvilinearlength (CL) for the W-shaped cells are longer than that for the baseline(CL=4.0 mm) and modified patterns (CL=4.5 mm). FIG. 21B shows that whenthe stent is deployed at 3.5 mm, the linear ring struts form interiorangles θ3 between each other. As can be seen from the figures, theinterior angle θ3 for the CL=5.0 mm is markedly smaller than theinterior angle θ1 for CL=4.0 mm and the interior angle θ2 for CL=4.5 mm.The decrease in interior angle is due to the increase in curvilinearlength and indicates that the risk of post-deployment recoil couldincrease. Therefore, it will be appreciated that CL should be carefullyselected to be within a range that will increase fracture resistanceduring deployment without unduly increasing the potential forpost-deployment recoil.

In some embodiments, a stent having a substrate made entirely of PLLAand configured to be crimped to an outer diameter of about 1.3 mm andexpanded to an outer diameter of at least 3.5 mm and more narrowly toabout 3.8 mm, has a strut pattern of W-shaped closed cells in an offsetbrick arrangement with a curvilinear length (CL) that is greater thanabout 4.0 mm and less than about 5.0 mm, and more narrowly at or about4.5 mm. The phrase “substrate” refers to the material at the core of thestructural element of the stent scaffold and does not include anycoatings of other material deposited after formation of the stentscaffold. The substrate can be formed from an extruded PLLA tube having0.64 mm (0.025 inch) ID and 1.7 mm (0.066 inch) OD, which is thenradially expanded by blow molding to an outer diameter of 3.5 mm to makea precursor tube out of which the scaffold pattern is cut. In furtherembodiments, the stent can have a coating over the substrate. Thecoating can contain any one of a mixture of the polymers, therapeuticagents, solvents and other substances described above. In furtherembodiments, the scaffold of the stent includes a plurality ofundulating rings formed linear ring struts and U-shaped hinge elements.The rings are arranged axially to form the tubular scaffold. For eachpair of rings, one ring is connected to a second ring by exactly threelinear link struts oriented axially so as to form exactly three W-shapedclosed cells arranged circumferentially. Each of the W-shaped closedcells having a curvilinear length (CL) that is greater than about 4.0 mmand less than about 5.0 mm, and more narrowly at or about 4.5 mm. Sinceeach ring corresponds to exactly three W-shaped closed cells arrangedcircumferentially, each ring can have an overall curvilinear length thatis about 12.0 mm to about 15.0 mm, and more narrowly at or about 13.5mm. The overall curvilinear length of the ring is the sum of curvilinearlengths of the three W-shaped closed cells.

While several particular forms of the invention have been illustratedand described, it will also be apparent that various modifications canbe made without departing from the scope of the invention. It is alsocontemplated that various combinations or subcombinations of thespecific features and aspects of the disclosed embodiments can becombined with or substituted for one another in order to form varyingmodes of the invention. Accordingly, it is not intended that theinvention be limited, except as by the appended claims.

What is claimed is:
 1. An endoprosthesis comprising: a tubular networkof struts cut from a radially expanded PLLA polymer tube, the tubularnetwork comprising W-shaped closed cells, each W-shaped closed cellabutting six other W-shaped closed cells, each W-shaped closed cellcomprising ring struts, link struts, U-shaped hinge elements, andY-shaped hinge elements, the ring struts forming a plurality of ringstructures, each ring structure connected to another one of the ringstructures by at least one link strut, each U-shaped hinge elementconnecting exactly two ring struts to each other, the U-shaped hingeelement being tangent to the two ring struts, each Y-shaped hingeelement connecting a link strut to exactly two ring struts, wherein thetubular network has an outer diameter of about 3.5 mm and is configuredto be crimped to an outer diameter of about 1.3 mm and expanded to anouter diameter of about 3.8 mm without substantial damage to theU-shaped hinge elements, and wherein each ring structure has an overallcurvilinear length from about 12 mm to about 15 mm.
 2. Theendoprosthesis of claim 1, wherein when the tubular network is at about3.5 mm in outer diameter, each of the ring struts are at a selectedangle relative to a link strut, the selected angle being from about 40degrees to about 50 degrees.
 3. The endoprosthesis of claim 1, whereinwhen the tubular network is at about 3.5 mm in outer diameter, one ofthe link struts is connected to a first ring strut and a second ringstrut, the first and second ring struts connected to opposite ends ofthe link strut, the first ring strut oriented at about 40 degreesrelative to the link strut, the second ring strut oriented at about 50degrees relative to the link strut.
 4. The endoprosthesis of claim 3,wherein the first ring strut is connected to a first U-shaped hingeelement and the second ring strut is connected to a second U-shapedhinge element circumferentially offset from the first U-shaped hingeelement when the tubular network is at about 3.5 mm in outer diameter.5. The endoprosthesis of claim 1, wherein each W-shaped closed cell hasa perimeter that includes eight of the ring struts and two of the linkstruts.
 6. The endoprosthesis of claim 1, wherein each W-shaped closedcell has a perimeter that includes four of the U-shaped hinge elementsand six of the Y-shaped hinge elements.
 7. The endoprosthesis of claim1, wherein each ring structure is connected to an adjacent ringstructure by exactly three of the link struts so as to form a ringstructure pair, there being a plurality of ring structure pairs andthere being exactly three W-shaped closed cells within each of the ringstructure pairs.
 8. The endoprosthesis of claim 7, wherein each of theW-shaped closed cells within each ring structure pair has a curvilinearlength from about 4 mm to about 5 mm.
 9. A method of making anendoprosthesis, the method comprising: cutting a radially expanded PLLApolymer tube to form a tubular network of struts, the tubular networkcomprising W-shaped closed cells, each W-shaped closed cell abutting sixother W-shaped closed cells, each W-shaped closed cell comprising ringstruts, link struts, U-shaped hinge elements, and Y-shaped hingeelements, the ring struts forming a plurality of ring structures, eachring structure connected to another one of the ring structures by atleast one link strut, each U-shaped hinge element connecting exactly tworing struts to each other, the U-shaped hinge element being tangent tothe two ring struts, each Y-shaped hinge element connecting a link strutto exactly two ring struts, wherein the tubular network has an outerdiameter of about 3.5 mm and is configured to be crimped to an outerdiameter of about 1.3 mm and expanded to an outer diameter of about 3.8mm without substantial damage to the U-shaped hinge elements, andwherein each ring structure has an overall curvilinear length from about12 mm to about 15 mm.
 10. The method of claim 9, further comprisingradially expanding a stock tube of PLLA polymer from an outer diameterof about 1.7 mm to an outer diameter of 3.5 mm in order to form theradially expanded PLLA polymer tube at a temperature above Tg of thePLLA polymer.
 11. The method of claim 10, further comprising crimpingthe tubular network onto a catheter, the crimping comprising reducingthe tubular network from an outer diameter of about 3.5 mm to a finisheddiameter of 1.3 mm.
 12. The method of claim 9, wherein when the tubularnetwork is at an outer diameter of 3.5 mm, each of the ring struts areat a selected angle relative to a link strut, the selected angle beingfrom about 40 degrees to about 50 degrees.
 13. The method of claim 9,wherein when the tubular network is at about 3.5 mm in outer diameter,one of the link struts is connected to a first ring strut and a secondring strut, the first and second ring struts connected to opposite endsof the link strut, the first ring strut oriented at about 40 degreesrelative to the link strut, the second ring strut oriented at about 50degrees relative to the link strut.
 14. The method of claim 13, whereinthe first ring strut is connected to a first U-shaped hinge element andthe second ring strut is connected to a second U-shaped hinge elementcircumferentially offset from the first U-shaped hinge element when thetubular network is at about 3.5 mm in outer diameter.
 15. The method ofclaim 9, wherein each W-shaped closed cell has a perimeter that includeseight of the ring struts and two of the link struts.
 16. The method ofclaim 9, wherein each W-shaped closed cell has a perimeter that includesfour of the U-shaped hinge elements and six of the Y-shaped hingeelements.
 17. The method of claim 9, wherein each ring structure isconnected to an adjacent ring structure by exactly three of the linkstruts so as to form a ring structure pair, there being a plurality ofring structure pairs and there being exactly three W-shaped closed cellswithin each of the ring structure pairs.
 18. The method of claim 17,wherein each of the W-shaped closed cells within each ring structurepair has a curvilinear length from about 4 mm to about 5 mm.